This invention relates to a method for producing a WC-Co-Cr alloy suitable for use as a hard non-corrosive alloy. The method involves use of submicron tungsten carbide and the use of chromium carbide as the source of chromium.
Wc-Co-Cr coatings have been commercially used for several years in highly corrosive and wear environments. These alloys contain typically 8-10 by weight cobalt, 3-4% by weight chromium, 4.5-5.5% by weight carbon and the balance being tungsten. Some of the newer alloys of this type such as those described in U.S. Pat. Nos. 4,626,476 and 4,588,608 contain combinations of 4-18% by weight cobalt, 2-11.5% by weight chromium, and 3.5% by weight carbon. These alloys have been developed with the idea of improving coating toughness and its resistance to high residual stresses (resulting from thermal expansion mismatch). However there is still an increasng demand for coatings with better internal strength and wear characteristics without sacrificing corrosion. It has become more desirable also to have smoother "as sprayed" deposits which require minimal finishing.
Coatings of WC-Co-Cr alloys derive their acclaimed properties from chemistry control and by controlling the spraying method. For instance it is widely accepted in the industry that Detonation gun (D-gun) deposits are significantly superior than the conventional and the high velocity plasma sprayed deposits for "hard coatings" (carbide based). It is well known also that coating strength is determined by the amount of carbide in the alloy. The greater the amount of WC, the stronger is the coating, provided it can be held together by a matrix. Cobalt has been traditionally developed as an ideal matrix material for WC-based alloys. As cobalt content is increased, the coating toughness increases proportionately. Chromium contributes towards improved corrosion. It also combines with W and C to improve wear performance by forming complex Cr-W-C carbides.